1. Field of the Invention
The present invention relates to high-throughput isolation and quantification of mRNA from whole blood. More particularly, this invention relates to a method and device for isolating and amplifying mRNA using combinations of leukocyte filters attached to oligo(dT)-immobilized multi-well plates.
2. Description of the Related Art
Research in the field of molecular biology has revealed that the genetic origin and functional activity of a cell can be deduced from the study of its ribonucleic acid (RNA). This information may be of use in clinical practice, to diagnose infections, to detect the presence of cells expressing oncogenes, to detect familial disorders, to monitor the state of host defense mechanisms and to determine the HLA type or other marker of identity. RNA exists in three functionally different forms: ribosomal RNA (rRNA), transfer RNA (tRNA) and messenger RNA (mRNA). Whereas stable rRNA and tRNA are involved in catalytic processes in translation, mRNA molecules carry genetic information. Only about 1-5% of the total RNA consists of mRNA, about 15% of tRNA and about 80% of rRNA.
mRNA is an important diagnostic tool, particularly when it is used to quantitatively observe up- or down-regulation of genes. Human peripheral blood is an excellent clinical resource for mRNA analysis. The detection of specific chimeric mRNA in blood, for example, indicates the presence of abnormal cells and is used in molecular diagnostics for chronic myelogenous leukemia (CML) (Kawasaki E. S., Clark S. S., Coyne M. Y., Smith S. D., Champlin R., Witte O. N., and McCormick F. P. 1988. Diagnosis of chronic myeloid and acute lymphocytic leukemias by detection of leukemia-specific mRNA sequences amplified in vitro. Proc. Natl. Acad. Sci. USA 85:5698-5702, Pachmann K., Zhao S., Schenk T., Kantaijian H., El-Naggar A. K., Siciliano M. J., Guo J. Q., Arlinghaus R. B., and Andreeff M. 2001. Expression of bcr-able mRNA individual chronic myelogenous leukaemia cells as determined by in situ amplification. Br. J. Haematol. 112:749-59). Micrometastatic cancer cells can also be detected in blood by measuring cancer-specific mRNA, such as carcinoembryonic antigen (CEA) for colon cancer, prostate specific antigen (PSA) for prostate cancer, thyroglobulin for thyroid cancer (Wingo S. T., Ringel M. D., Anderson J. S., Patel A. D., Lukes Y. D., Djuh Y. Y., Solomon B., Nicholson D., Balducci-Silano P. L., Levine M. A., Francis G. L., and Tuttle R. M. 1999. Quantitative reverse transcription-PCR measurement of thyroglobulin mRNA in peripheral blood of healthy subjects. Clin. Chem. 45:785-89), and tyrosinase for melanoma (Pelkey T. J., Frierson H. F. Jr., and Bruns D. E. 1996. Molecular and immunological detection of circulating tumor cells and micrometastasis from solid tumors. Clin. Chem. 42:1369-81). Moreover, as the levels of these cancer-specific mRNA can change following treatment, quantification of specific mRNA provides for a useful indicator during treatment follow-up.
As blood contains large quantities of non-nucleated erythrocytes (approximately 5 million cells/μL) compared to leukocytes (approximately 5000 leukocytes/μL), the isolation of granulocytes or lymphocytes from whole blood is commonly performed as the first step in mRNA analysis. However, due to inconsistencies in the recovery of specific subsets of leukocytes among different samples, the number of isolated leukocytes is determined for each sample and results are expressed as the quantity of mRNA per leukocytes, not mRNA/μL blood. Moreover, mRNA quantities may change during lengthy isolation processes. While no method exists for the isolation of cancer cells from blood, gene amplification technologies enable the identification and quantification of specific mRNA levels even from a pool of different genes, making whole blood an ideal material for mRNA analysis when gene-specific primers and probes are available.
The scientific community is facing a huge problem of institute-to-institute and experiment-to-experiment variation in gene expression analysis, because of the lack of standardization. Although recent gene amplification technologies provide an absolute quantity of template DNA, these values cannot be converted to the amounts of the gene in the original materials, due to the lack of information of the yield of RNA recovery and the efficiency of cDNA synthesis in each sample. Total RNA is frequently used as a standardization marker for mRNA quantitation, and results are typically expressed as the amounts of genes per μg total RNA. However, it must be emphasized that total RNA does not represent mRNA, because the fraction of mRNA is only 1-5% of total RNA, and mRNA volume varies even when the amounts of total RNA is identical. The yield of total RNA or mRNA also varies widely depending on which method is employed. Once RNA is extracted, the next step is the synthesis of cDNA, which itself can create uncertainty since existing methods do not indicate whether each RNA template creates a single copy of cDNA in each experiment. In order to avoid the above problems, relative quantitation is used widely by comparing the data of target genes to that of housekeeping genes or rRNA. However, the amounts of control genes are typically not consistent and may change during experiments. Moreover, this variation presents a serious problem for clinical diagnostics, since each clinical specimen is typically analyzed at a different point in time.
It is typically very difficult to isolate pure mRNA from whole blood because whole blood contains large amounts of RNAases (from granulocytes) and non-nucleated erythrocytes. Although various RNA extraction methods are available for whole blood applications (de Vries T. J., Fourkour A., Punt C. J., Ruiter D. J., and van Muijen G. N. 2000. Analysis of melanoma cells in peripheral blood by reverse transcription-polymerase chain reaction for tyrosinase and MART-1 after mononuclear cell collection with cell preparation tubes: a comparison with the whole blood guanidinium isothiocyanate RNA isolation method. Melanoma Research 10:119-26, Johansson M., Pisa E. K., Tormanen V., Arstrand K., and Kagedal Bl. 2000. Quantitative analysis of tyrosinase transcripts in blood. Clin. Chem. 46:921-27, Wingo S. T., Ringel M. D., Anderson J. S., Patel A. D., Lukes Y. D., Djuh Y. Y., Solomon B., Nicholson D., Balducci-Silano P. L., Levine M. A., Francis G. L., and Tuttle R. M. 1999. Quantitative reverse transcription-PCR measurement of thyroglobulin mRNA in peripheral blood of healthy subjects. Clin. Chem. 45:785-89), the assay procedures are labor-intensive, require several rounds of centrifugation, and involve careful handling that is essential in eliminating ribonuclease activities.
Consequently, there exists a need for a quick and easy method and device for isolating and quantifying large quantities of mRNA from whole blood. Specifically, there exists a need for a high throughput, whole blood-derived mRNA-processing technology with reproducible recovery and a seamless process to gene amplification.